In recent years, a transistor including an oxide semiconductor has attracted attention. An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used for a semiconductor of a transistor in a large display device. In addition, there is an advantage in a transistor including an oxide semiconductor that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized.
It is known that a transistor including an oxide semiconductor has an extremely low leakage current in an off state. For example, a low-power CPU and the like utilizing the characteristics that a leakage current of the transistor including an oxide semiconductor is low is disclosed (see Patent Document 1).